High efficiency optical devices

ABSTRACT

Optical devices such as backlight assemblies, according to the present invention include a multilayer optical film in which at least one of the layers comprises an oriented birefringent polymer. The multilayer optical film exhibits low absorptivity and can reflect light approaching at shallow angles as well as normal to the film.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/700,137, filed Nov. 3, 2003 now U.S. Pat. No. 6,924,014 which is acontinuation of U.S. application Ser. No. 09/777,455, filed Feb. 6, 2001and issued as U.S. Pat. No. 6,641,883, which is a divisional of U.S.application Ser. No. 09/461,245, filed Dec. 15, 1999 and issued as U.S.Pat. No. 6,210,785, which is a continuation of U.S. application Ser. No.08/494,366, filed Jun. 26, 1995 and issued as U.S. Pat. No. 6,080,467.

FIELD OF THE INVENTION

The present invention relates to the field of optical devices. Moreparticularly, the present invention relates to optical devices employingmulti-layer optical film as reflectors and/or polarizers.

BACKGROUND OF THE INVENTION

Optical devices employing reflectors are used, for example, in displaysfor laptop computers, hand-held calculators, digital watches and similardevices as well as illuminated signs, light pipes, backlight assembliesand many other devices.

Conventional reflectors, including pigmented surfaces, silvered mirrors,polished metallic or metallized surfaces, etc. suffer from a number ofdisadvantages in many applications. The conventional reflectors sufferfrom relatively high absorbance of light incident on their surfaces,typically absorbing about 4–10% of the light incident on them. As aresult, the amount of light remaining after each reflection is less thanthat initially provided. In devices in which multiple reflections areencountered, the overall output of the optical device can besubstantially limited. In addition, many of the conventional reflectorsare too bulky and/or heavy for many of the applications, particularly inlaptop computer displays and other portable devices.

Many optical devices use polarizers, either alone or in combination withreflectors, to provide light having substantially one plane ofpolarization. Polarized light is especially useful in conjunction withliquid crystal (LC) displays used in many portable devices such aslaptop computers and watches, because the LC displays rely on polarizedlight passing through the LC to display information to a viewer.

Polarizers can be generally categorized as either absorptive orreflective. Typical absorptive polarizers are oriented dyed polymerfilms, while typical reflective polarizers are tilted thin filmpolarizers, also known as MacNeille polarizers. Absorptive polarizersdo, of course, contribute to the absorptive losses of optical devices inwhich they are used, thereby limiting the output of those devices.

The absorptive losses of known reflectors and polarizers become muchmore important when the optical devices are used with a brightnessenhancement film such as micro-replicated brightness enhancement film orany other type of reflective polarizer which causes light to typicallytravel through several reflections, thereby amplifying absorptive losseswith every reflection. In the highest gain configurations, for, e.g., asingle sheet of brightness enhancement film in combination with areflective polarizer and back reflector, or two sheets of orthogonallycrossed sheets of brightness enhancement film, the effective absorptivelosses can reduce the total potential light output of an optical displayby 10–30%.

This principle of absorptive losses also applies to optical devicesemploying non-totally internally reflecting surfaces. One example is anoptical wedge in which light is directed into a structure havingconverging reflective surfaces. Optical wedges will typically reflectlight many times before it exits the device. With each reflection,however, some of the light which entered the wedge is absorbed byconventional reflectors. As a result, the amount of light exiting thedevice is typically substantially less than the light entering thedevice.

Another optical device typically employing reflective surfaces is anilluminated sign which relies on a finite number of light sources andmultiple reflections within an optical cavity to disperse the light toilluminate the surface of a sign in a generally uniform manner. Toovercome the problems associated with absorptive losses, many signstypically employ numerous light sources, thereby increasing the cost tomanufacture and operate the signs.

Yet another optical device which is limited by absorption losses is alight pipe in which light enters the pipe and is reflected along itslength numerous times before exiting at a desired location. Eachreflection results in some absorption when conventional reflectors areused, thereby limiting throughput of the light pipe.

To overcome some of the problems of weight, bulk and absorption ofconventional reflectors, multi-layered polymer films have been used toreflect and/or polarize light. Such polymeric films are, however,subject to a number of other disadvantages including iridescence, aswell as poor reflectivity when off-axis light approaches the surface ofthe film. The off-axis light is typically transmitted through the films,rather than being reflected, thereby resulting in transmissive lossesrather than absorptive losses. Whether light is lost through absorptionor transmission, however, the output of the optical device is limited.

Other problems with known multi-layer polymer films used to providereflectors and/or polarizers is that the materials and methods used tomanufacture the films presents serious problems due to poor opticaltransmission, extrudibility, and high costs.

SUMMARY OF THE INVENTION

Optical devices according to the present invention include a multilayeroptical film. Optical devices incorporating multilayer optical filmaccording to the present invention enjoy many advantages due to the lowabsorptivity of the film and its ability to reflect light approaching atshallow angles as well as normal to the film.

In those situations where complete reflectivity is desired, opticaldevices employing a multilayer optical film according to the presentinvention can reflect over 99% of the light striking the surface of thefilm.

If a reflective polarizer is desired, the optical devices can beconstructed with a multilayer optical film which transmits a significantamount of light having one plane of polarization while reflecting asignificant amount of light having an orthogonally orientedpolarization. A further advantage is that the relative percentages oftransmitted/reflected light can be largely controlled by the multilayeroptical film used in the present invention.

As a result of the unique properties of the multilayer optical film,optical devices according to the present invention are highly efficientat reflecting and transporting light and/or transmitting light of onepolarization, whether the light is incident normal to the film surfaceor off-axis.

Another advantage of optical devices employing multilayer optical filmaccording to the present invention which rely on reflection to transportlight is that the devices need not have symmetry to reduce the number ofreflections needed to transmit light due to the low absorptivity of themultilayer optical film.

Yet another advantage of optical devices employing multilayer opticalfilms according to the present invention is their relatively low weightas compared to many conventional reflectors and/or polarizers.

Still another advantage of optical devices employing multilayer opticalfilms according to the present invention is that because the film isrelatively thin as compared to many conventional reflectors and/orpolarizers, the optical devices can be manufactured to occupy limitedspace in a system employing the device.

Additional features and advantages of optical devices according to thepresent invention will be apparent upon reading the detailed descriptionof illustrative embodiments below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b are diagrammatical views of the multilayer opticalfilm of the present invention.

FIG. 2 depicts a two layer stack of films forming a single interface.

FIGS. 3–6, 7A and 7B depict the optical performance of multilayeroptical films described in Examples 1–5.

FIG. 8 is a graphical representation illustrating the relationshipbetween the number of reflections experienced by a ray of light (x-axis)as compared to the relative intensity of the light ray (y-axis) forreflective surfaces made of multilayer optical film and a standardreflector.

FIG. 9 is schematic cross-sectional diagram of an alternate opticaldevice according to the present invention.

FIG. 10 is a perspective view of the optical device of FIG. 9 in whichat least one surface of the device is intended to display a message.

FIG. 11 is a schematic cross-sectional diagram of a converging wedgeoptical device according to the present invention.

FIG. 12 is a schematic cross-sectional diagram of a diverging wedgeoptical device according to the present invention.

FIG. 13 is a schematic cross-sectional diagram of a light pipe employingmultilayer optical films according to the present invention.

FIG. 14 is a schematic cross-sectional diagram of the device of FIG. 13,taken along a plane transverse to the longitudinal axis of the lightpipe.

FIG. 15 is a perspective view of one illustrative optical deviceconstructed using multilayer optical films according to the presentinvention.

FIGS. 16 and 17 show reflectivity versus angle curves for a uniaxialbirefringent system in a medium of index 1.60.

FIG. 18 shows reflectivity versus angle curves for a uniaxialbirefringent system in a medium of index 1.0.

FIGS. 19, 20, and 21 show various relationships between in-plane indicesand z-index for a uniaxial birefringent system.

FIG. 22 shows off axis reflectivity versus wavelength for two differentbiaxial birefringent systems.

FIG. 23 shows the effect of introducing a y-index difference in abiaxial birefringent film with a large z-index difference.

FIG. 24 shows the effect of introducing a y-index difference in abiaxial birefringent film with a smaller z-index difference.

FIG. 25 shows a contour plot summarizing the information from FIGS. 23and 24.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE INVENTION

I. Multilayer Optical Film

The optical devices described herein rely on the unique and advantageousproperties of multilayer optical films. The advantages, characteristicsand manufacturing of such films are most completely described in U.S.Pat. No. 5,882,774, which is incorporated herein by reference. Themultilayer optical film is useful, for example, as highly efficientmirrors and/or polarizers. A relatively brief description of theproperties and characteristics of the multilayer optical film ispresented below followed by a description of illustrative embodiments ofbacklight systems using the multilayer optical film according to thepresent invention.

Multilayer optical films as used in conjunction with the presentinvention exhibit relatively low absorption of incident light, as wellas high reflectivity for off-axis as well as normal light rays. Theseproperties generally hold whether the films are used for pure reflectionor reflective polarization of light. The unique properties andadvantages of the multi-layer optical film provides an opportunity todesign highly efficient backlight systems which exhibit low absorptionlosses when compared to known backlight systems.

An exemplary multilayer optical film of the present invention asillustrated in FIGS. 1A and 1B includes a multilayer stack 10 havingalternating layers of at least two materials 12 and 14. At least one ofthe materials has the property of stress induced birefringence, suchthat the index of refraction (n) of the material is affected by thestretching process. FIG. 1A shows an exemplary multilayer stack beforethe stretching process in which both materials have the same index ofrefraction. Light ray 13 experiences no index of refraction change andpasses through the stack. In FIG. 1B, the same stack has been stretched,thus increasing the index of refraction of material 12. The differencein refractive index at each boundary between layers will cause part ofray 15 to be reflected. By stretching the multilayer stack over a rangeof uniaxial to biaxial orientation, a film is created with a range ofreflectivities for differently oriented plane-polarized incident light.The multilayer stack can thus be made useful as reflective polarizers ormirrors.

Multilayer optical films constructed according to the present inventionexhibit a Brewster angle (the angle at which reflectance goes to zerofor light incident at any of the layer interfaces) which is very largeor is nonexistent. In contrast, known multilayer polymer films exhibitrelatively small Brewster angles at layer interfaces, resulting intransmission of light and/or undesirable iridescence. The multilayeroptical films according to the present invention, however, allow for theconstruction of mirrors and polarizers whose reflectivity for ppolarized light decrease slowly with angle of incidence, are independentof angle of incidence, or increase with angle of incidence away from thenormal. As a result, multilayer stacks having high reflectivity for boths and p polarized light over a wide bandwidth, and over a wide range ofangles can be achieved.

FIG. 2 shows two layers of a multilayer stack, and indicates the threedimensional indices of refraction for each layer. The indices ofrefraction for each layer are n1x, n1y, and n1z for layer 102, and n2x,n2y, and n2z for layer 104. The relationships between the indices ofrefraction in each film layer to each other and to those of the otherlayers in the film stack determine the reflectance behavior of themultilayer stack at any angle of incidence, from any azimuthaldirection. The principles and design considerations described in U.S.Pat. No. 5,882,774 can be applied to create multilayer stacks having thedesired optical effects for a wide variety of circumstances andapplications. The indices of refraction of the layers in the multilayerstack can be manipulated and tailored to produce the desired opticalproperties.

Referring again to FIG. 1B, the multilayer stack 10 can include tens,hundreds or thousands of layers, and each layer can be made from any ofa number of different materials. The characteristics which determine thechoice of materials for a particular stack depend upon the desiredoptical performance of the stack. The stack can contain as manymaterials as there are layers in the stack. For ease of manufacture,preferred optical thin film stacks contain only a few differentmaterials.

The boundaries between the materials, or chemically identical materialswith different physical properties, can be abrupt or gradual. Except forsome simple cases with analytical solutions, analysis of the latter typeof stratified media with continuously varying index is usually treatedas a much larger number of thinner uniform layers having abruptboundaries but with only a small change in properties between adjacentlayers.

The preferred multilayer stack is comprised of low/high index pairs of,film layers, wherein each low/high index pair of layers has a combinedoptical thickness of ½ the center wavelength of the band it is designedto reflect. Stacks of such films are commonly referred to as quarterwavestacks. For multilayer optical films concerned with the visible and thenear infrared wavelengths, a quarterwave stack design results in each ofthe layers in the multilayer stack having an average thickness of notmore than 0.5 microns.

In those applications where reflective films (e.g. mirrors) are desired,the desired average transmission for light of each polarization andplane of incidence generally depends upon the intended use of thereflective film. One way to produce a multilayer mirror film is tobiaxially stretch a multilayer stack which contains a birefringentmaterial as the high index layer of the low/high index pair. For a highefficiency reflective film, average transmission along each stretchdirection at normal incidence over the visible spectrum (400–700 nm) isdesirably less than 10% (reflectance greater than 90%), preferably lessthan 5% (reflectance greater than 95%), more preferably less than 2%(reflectance greater than 98%) , and even more preferably less than 1%(reflectance greater than 99%). The average transmission at 60 degreesfrom the normal from 400–700 nm is desirably less than 20% (reflectancegreater than 80%), preferably less than 10% (reflectance greater than90%), more preferably less than 5% (reflectance greater than 95%), andeven more preferably less than 2% (reflectance greater than 98%), andeven more preferably less than 1% (reflectance greater than 99%).

In addition, asymmetric reflective films may be desirable for certainapplications. In that case, average transmission along one stretchdirection may be desirably less than, for example, 50%, while theaverage transmission along the other stretch direction may be desirablyless than, for example 20%, over a bandwidth of, for example, thevisible spectrum (400–700 nm), or over the visible spectrum and into thenear infrared (e.g, 400–850 nm).

Multilayer optical films can also be designed to operate as reflectivepolarizers. One way to produce a multilayer reflective polarizer is touniaxially stretch a multilayer stack which contains a birefringentmaterial as the high index layer of the low/high index pair. Theresulting reflective polarizers have high reflectivity for light withits plane of polarization parallel to one axis (in the stretchdirection) for a broad range of angles of incidence, and simultaneouslyhave low reflectivity and high transmissivity for light with its planeof polarization parallel to the other axis (in the non-stretchdirection) for a broad range of angles of incidence. By controlling thethree indices of refraction of each film, nx, ny and nz, the desiredpolarizer behavior can be obtained.

For many applications, the ideal reflecting polarizer has highreflectance along one axis (the so-called extinction axis) and zeroreflectance along the other (the so-called transmission axis), at allangles of incidence. For the transmission axis of a polarizer, itgenerally desirable to maximize transmission of light polarized in thedirection of the transmission axis over the bandwidth of interest andalso over the range of angles of interest.

The average transmission at normal incidence for a polarizer in thetransmission axis across the visible spectrum (400–700 nm for abandwidth of 300 nm) is desirably at least 50%, preferably at least 70%,more preferably at least 85%, and even more preferably at least 90%. Theaverage transmission at 60 degrees from the normal (measured along thetransmission axis for p-polarized light) for a polarizer from 400–700 nmis desirably at least 50%, preferably at least 70%, more preferably atleast 80%, and even more preferably at least 90%.

The average transmission for a multilayer reflective polarizer at normalincidence for light polarized in the direction of the extinction axisacross the visible spectrum (400–700 nm for a bandwidth of 300 nm) isdesirably at less than 50%, preferably less than 30%, more preferablyless than 15%, and even more preferably less than 5%. The averagetransmission at 60 degrees from the normal (measured along thetransmission axis for p-polarized light) for a polarizer for lightpolarized in the direction of the extinction axis from 400–700 nm isdesirably less than 50%, preferably less than 30%, more preferably lessthan 15%, and even more preferably less than 5%.

For certain applications, high reflectivity for p-polarized light withits plane of polarization parallel to the transmission axis atoff-normal angles are preferred. The average reflectivity for lightpolarized along the transmission axis should be more than 20% at anangle of at least 30 degrees from the normal.

In addition, although reflective polarizing films and asymmetricreflective films are discussed separately herein, it should beunderstood that two or more of such films could be provided to reflectsubstantially all light incident on them (provided they are properlyoriented with respect to each other to do so). This construction istypically desired when the multilayer optical film is used as areflector in a backlight system according to the present invention.

If some reflectivity occurs along the transmission axis, the efficiencyof the polarizer at off-normal angles may be reduced. If thereflectivity along the transmission axis is different for variouswavelengths, color may be introduced into the transmitted light. One wayto measure the color is to determine the root mean square (RMS) value ofthe transmissivity at a selected angle or angles over the wavelengthrange of interest. The % RMS color, C_(RMS), can be determined accordingto the equation:

$C_{RMS} = \frac{\int_{\lambda 1}^{\lambda 2}{\left( \left( {T - \overset{\_}{T}} \right)^{2} \right)^{1/2}\ {\mathbb{d}\lambda}}}{\overset{\_}{T}\left( {{\lambda 2} - {\lambda 1}} \right)}$where the range λ1 to λ2 is the wavelength range, or bandwidth, ofinterest, T is the transmissivity along the transmission axis, and{overscore (T)} is the average transmissivity along the transmissionaxis in the wavelength range of interest. For applications where a lowcolor polarizer is desirable, the % RMS color should be less than 10%,preferably less than 8%, more preferably less than 3.5%, and even morepreferably less than 2% at an angle of at least 30 degrees from thenormal, preferably at least 45 degrees from the normal, and even morepreferably at least 60 degrees from the normal.

Preferably, a reflective polarizer combines the desired % RMS coloralong the transmission axis for the particular application with thedesired amount of reflectivity along the extinction axis across thebandwidth of interest. For polarizers having a bandwidth in the visiblerange (400–700 nm, or a bandwidth of 300 nm), average transmission alongthe extinction axis at normal incidence is desirably less than 40%, moredesirably less than 25%, preferably less than 15%, more preferably lessthan 5% and even more preferably less than 3%.

Materials Selection and Processing

With the design considerations described in the above mentioned U.S.Pat. No. 5,882,774, one of ordinary skill will readily appreciate that awide variety of materials can be used to form multilayer reflectivefilms or polarizers according to the invention when processed underconditions selected to yield the desired refractive index relationships.The desired refractive index relationships can be achieved in a varietyof ways, including stretching during or after film formation (e.g., inthe case of organic polymers), extruding (e.g., in the case of liquidcrystalline materials), or coating. In addition, it is preferred thatthe two materials have similar rheological properties (e.g., meltviscosities) such that they can be co-extruded.

In general, appropriate combinations may be achieved by selecting, asthe first material, a crystalline or semi-crystalline material,preferably a polymer. The second material, in turn, may be crystalline,semi-crystalline, or amorphous. The second material may have abirefringence opposite of the first material. Or, the second materialmay have no birefringence, or less birefringence than the firstmaterial.

Specific examples of suitable materials include polyethylene naphthalate(PEN) and isomers thereof (e.g., 2,6-, 1,4-, 1,5-, 2,7-, and 2,3-PEN),polyalkylene terephthalates (e.g., polyethylene terephthalate,polybutylene terephthalate, and poly-1,4-cyclohexanedimethyleneterephthalate), polyimides (e.g., polyacrylic imides), polyetherimides,atactic polystyrene, polycarbonates, polymethacrylates (e.g.,polyisobutyl methacrylate, polypropylmethacrylate,polyethylmethacrylate, and polymethylmethacrylate), polyacrylates (e.g.,polybutylacrylate and polymethylacrylate), syndiotactic polystyrene(sPS), syndiotactic poly-alpha-methyl styrene, syndiotacticpolydichlorostyrene, copolymers and blends of any of these polystyrenes,cellulose derivatives (e.g., ethyl cellulose, cellulose acetate,cellulose propionate, cellulose acetate butyrate, and cellulosenitrate), polyalkylene polymers (e.g., polyethylene, polypropylene,polybutylene, polyisobutylene, and poly(4-methyl)pentene), fluorinatedpolymers (e.g., perfluoroalkoxy resins, polytetrafluoroethylene,fluorinated ethylene-propylene copolymers, polyvinylidene fluoride, andpolychlorotrifluoroethylene), chlorinated polymers (e.g., polyvinylidenechloride and polyvinylchloride), polysulfones, polyethersulfones,polyacrylonitrile, polyamides, silicone resins, epoxy resins,polyvinylacetate, polyetheramides, ionomeric resins, elastomers (e.g.,polybutadiene, polyisoprene, and neoprene), and polyurethanes. Alsosuitable are copolymers, e.g., copolymers of PEN (e.g., copolymers of2,6-, 1,4-, 1,5-, 2,7-, and/or 2,3-naphthalene dicarboxylic acid, oresters thereof, with (a) terephthalic acid, or esters thereof; (b)isophthalic acid, or esters thereof; (c) phthalic acid, or estersthereof, (d) alkane glycols; (e) cycloalkane glycols (e.g., cyclohexanedimethane diol); (f) alkane dicarboxylic acids; and/or (g) cycloalkanedicarboxylic acids (e.g., cyclohexane dicarboxylic acid)), copolymers ofpolyalkylene terephthalates (e.g., copolymers of terephthalic acid, oresters thereof, with (a) naphthalene dicarboxylic acid, or estersthereof; (b) isophthalic acid, or esters thereof, (c) phthalic acid, oresters thereof; (d) alkane glycols; (e) cycloalkane glycols (e.g.,cyclohexane dimethane diol); (f) alkane dicarboxylic acids; and/or (g)cycloalkane dicarboxylic acids (e.g., cyclohexane dicarboxylic acid)),and styrene copolymers (e.g., styrene-butadiene copolymers andstyrene-acrylonitrile copolymers), 4,4′-bibenzoic acid and ethyleneglycol. In addition, each individual layer may include blends of two ormore of the above-described polymers or copolymers (e.g., blends of sPSand atactic polystyrene). The coPEN described may also be a blend ofpellets where at least one component is a polymer based on naphthalenedicarboxylic acid and other components are other polyesters orpolycarbonates, such as a PET, a PEN or a co-PEN.

Particularly preferred combinations of layers in the case of polarizersinclude PEN/co-PEN, polyethylene terephthalate (PET)/co-PEN, PEN/sPS,PET/sPS, PEN/Eastar, and PET/Eastar, where “co-PEN” refers to acopolymer or blend based upon naphthalene dicarboxylic acid (asdescribed above) and Eastar is polycyclohexanedimethylene terephthalatecommercially available from Eastman Chemical Co.

Particularly preferred combinations of layers in the case of reflectivefilms include PET/Ecdel, PEN/Ecdel, PEN/sPS, PEN/THV, PEN/co-PET, andPET/sPS, where “co-PET” refers to a copolymer or blend based uponterephthalic acid (as described above), Ecdel is a thermoplasticpolyester commercially available from Eastman Chemical Co., and THV is afluoropolymer commercially available from Minnesota Mining andManufacturing Company, St. Paul, Minn.

The number of layers in the film is selected to achieve the desiredoptical properties using the minimum number of layers for reasons offilm thickness, flexibility and economy. In the case of both polarizersand reflective films, the number of layers is preferably less than10,000, more preferably less than 5,000, and even more preferably lessthan 2,000.

As discussed above, the ability to achieve the desired relationshipsamong the various indices of refraction (and thus the optical propertiesof the multilayer film) is influenced by the processing conditions usedto prepare the multilayer film. In the case of organic polymers whichcan be oriented by stretching, the films are generally prepared byco-extruding the individual polymers to form a multilayer film and thenorienting the film by stretching at a selected temperature, optionallyfollowed by heat-setting at a selected temperature. Alternatively, theextrusion and orientation steps may be performed simultaneously. In thecase of polarizers, the film is stretched substantially in one direction(uniaxial orientation), while in the case of reflective films the filmis stretched substantially in two directions (biaxial orientation).

The film may be allowed to dimensionally relax in the cross-stretchdirection from the natural reduction in cross-stretch (equal to thesquare root of the stretch ratio); it may simply be constrained to limitany substantial change in cross-stretch dimension; or it may be activelystretched in the cross-stretch dimension. The film may be stretched inthe machine direction, as with a length orienter, or in width using atenter.

The pre-stretch temperature, stretch temperature, stretch rate, stretchratio, heat set temperature, heat set time, heat set relaxation, andcross-stretch relaxation are selected to yield a multilayer film havingthe desired refractive index relationship. These variables areinter-dependent; thus, for example, a relatively low stretch rate couldbe used if coupled with, e.g., a relatively low stretch temperature. Itwill be apparent to one of ordinary skill how to select the appropriatecombination of these variables to achieve the desired multilayer film.In general, however, a stretch ratios in the range from 1:2 to 1:10(more preferably 1:3 to 1:7) in the stretch direction and from 1:0.2 to1:10 (more preferably from 1:0.3 to 1:7) orthogonal to the stretchdirection is preferred.

Suitable multilayer films may also be prepared using techniques such asspin coating (e.g., as described in Boese et al., J. Polym. Sci.: PartB, 30:1321 (1992)) for birefringent polyimides, and vacuum deposition(e.g., as described by Zang et. al., Appl. Phys. Letters, 59:823 (1991))for crystalline organic compounds; the latter technique is particularlyuseful for certain combinations of crystalline organic compounds andinorganic materials.

Exemplary multilayer reflective mirror films and multilayer reflectivepolarizers will now be described in the following examples.

EXAMPLE 1

(PEN:THV 500, 449, Mirror)

A coextruded film containing 449 layers was made by extruding the castweb in one operation and later orienting the film in a laboratoryfilm-stretching apparatus. A Polyethylene naphthalate (PEN) with anIntrinsic Viscosity of 0.53 dl/g (60 wt. % phenol/40 wt. %dichlorobenzene) was delivered by one extruder at a rate of 56 poundsper hour and THV 500 (a fluoropolymer available from Minnesota Miningand Manufacturing Company) was delivered by another extruder at a rateof 11 pounds per hour. The PEN was on the skin layers and 50% of the PENwas present in the two skin layers. The feedblock method was used togenerate 57 layers which was passed through three multipliers producingan extrudate of 449 layers. The cast web was 20 mils thick and 12 incheswide. The web was later biaxially oriented using a laboratory stretchingdevice that uses a pantograph to grip a square section of film andsimultaneously stretch it in both directions at a uniform rate. A 7.46cm square of web was loaded into the stretcher at about 100° C. andheated to 140° C. in 60 seconds. Stretching then commenced at 10%/sec(based on original dimensions) until the sample was stretched to about3.5×3.5. Immediately after the stretching the sample was cooled byblowing room temperature air at it.

FIG. 3 shows the transmission of this multilayer film. Curve (a) showsthe response at normal incidence, while curve (b) shows the response at60 degrees for p-polarized light.

EXAMPLE 2

(PEN:PMMA, 601, Mirror)

A coextruded film containing 601 layers was made on a sequentialflat-film-making line via a coextrusion process. PolyethyleneNaphthalate (PEN) with an Intrinsic Viscosity of 0.57 dl/g (60 wt. %phenol/40 wt. % dichlorobenzene) was delivered by extruder A at a rateof 114 pounds per hour with 64 pounds per hour going to the feedblockand the rest going to skin layers described below. PMMA (CP-82 from ICIof Americas) was delivered by extruder B at a rate of 61 pounds per hourwith all of it going to the feedblock. PEN was on the skin layers of thefeedblock. The feedblock method was used to generate 151 layers usingthe feedblock such as those described in U.S. Pat. No. 3,801,429, afterthe feedblock two symmetric skin layers were coextruded using extruder Cmetering about 30 pounds per hour of the same type of PEN delivered byextruder A. This extrudate passed through two multipliers producing anextrudate of about 601 layers. U.S. Pat. No. 3,565,985 describes similarcoextrusion multipliers. The extrudate passed through another devicethat coextruded skin layers at a total rate of 50 pounds per hour of PENfrom extruder A. The web was length oriented to a draw ratio of about3.2 with the web temperature at about 280° F. The film was subsequentlypreheated to about 310° F. in about 38 seconds and drawn in thetransverse direction to a draw ratio of about 4.5 at a rate of about 11%per second. The film was then heat-set at 440° F. with no relaxationallowed. The finished film thickness was about 3 mil.

As seen in FIG. 4, curve (a), the bandwidth at normal incidence is about350 nm with an average in-band extinction of greater than 99%. Theamount of optical absorption is difficult to measure because of its lowvalue, but is less than 1%. At an incidence angle of 50° from the normalboth s (curve (b)) and p-polarized (curve (c)) light showed similarextinctions, and the bands were shifted to shorter wavelengths asexpected. The red band-edge for s-polarized light is not shifted to theblue as much as for p-polarized light due to the expected largerbandwidth for s-polarized light, an due to the lower index seen by thep-polarized light in the PEN layers.

EXAMPLE 3

(PEN:PCTG, 449, Polarizer)

A coextruded film containing 481 layers was made by extruding the castweb in one operation and later orienting the film in a laboratoryfilm-stretching apparatus. The feedblock method was used with a 61 layerfeedblock and three (2×) multipliers. Thick skin layers were addedbetween the final multiplier and the die. Polyethylene naphthalate (PEN)with an intrinsic viscosity of 0.47 dl/g (60 wt. % phenol/40 wt. %dichlorobenzene) was delivered to the feedblock by one extruder at arate of 25.0 pounds per hour. Glycol modified polyethylene dimethylcyclohexane terephthalate (PCTG 5445 from Eastman) was delivered byanother extruder at a rate of 25.0 pounds per hour. Another stream ofPEN from the above extruder was added as skin layers after themultipliers at a rate of 25.0 pounds per hour. The cast web was 0.007inches thick and 12 inches wide. The web was layer uniaxially orientedusing a laboratory stretching device that uses a pantograph to grip asection of film and stretch it in one direction at a uniform rate whileit is allowed to freely relax in the other direction. The sample of webloaded was about 5.40 cm wide (the unconstrained direction) and 7.45 cmlong between the grippers of the pantograph. The web was loaded into thestretcher at about 100° C. and heated to 135° C. for 45 seconds.Stretching was then commenced at 20%/second (based on originaldimensions) until the sample was stretched to about 6:1 (based ongripper to gripper measurements). Immediately after stretching, thesample was cooled by blowing room temperature air at it. In the center,the sample was found to relax by a factor of 2.0.

FIG. 5 shows the transmission of this multilayer film where curve ashows transmission of light polarized in the non-stretch direction atnormal incidence, curve b shows transmission of p-polarized lightpolarized in the non-stretched direction at 60° incidence, and curve cshows the transmission of light polarized in the stretch direction atnormal incidence. Average transmission for curve a from 400–700 nm is89.7%, average transmission for curve b from 400–700 nm is 96.9%, andaverage transmission for curve c from 400–700 nm is 4.0%. % RMS colorfor curve a is 1.05%, and % RMS color for curve b is 1.44%.

EXAMPLE 4

(PEN:CoPEN, 601, Polarizer)

A coextruded film containing 601 layers was made on a sequentialflat-film-making line via a coextrusion process. A Polyethylenenaphthalate (PEN) with an intrinsic viscosity of 0.54 dl/g (60 wt %Phenol plus 40 wt % dichlorobenzene) was delivered by on extruder at arate of 75 pounds per hour and the coPEN was delivered by anotherextruder at 65 pounds per hour. The coPEN was a copolymer of 70 mole %2,6 naphthalene dicarboxylate methyl ester, 15% dimethyl isophthalateand 15% dimethyl terephthalate with ethylene glycol. The feedblockmethod was used to generate 151 layers. The feedblock was designed toproduce a stack of films having a thickness gradient from top to bottom,with a thickness ratio of 1.22 from the thinnest layers to the thickestlayers. The PEN skin layers were coextruded on the outside of theoptical stack with a total thickness of 8% of the coextruded layers. Theoptical stack was multiplied by two sequential multipliers. The nominalmultiplication ratio of the multipliers were 1.2 and 1.27, respectively.The film was subsequently preheated to 310° F. in about 40 seconds anddrawn in the transverse direction to a draw ratio of about 5.0 at a rateof 6% per second. The finished film thickness was about 2 mils.

FIG. 6 shows the transmission for this multilayer film. Curve a showstransmission of light polarized in the non-stretch direction at normalincidence, curve b shows transmission of p-polarized light at 60°incidence, and curve c shows transmission of light polarized in thestretch direction at normal incidence. Note the very high transmissionof p-polarized light in the non-stretch direction at both normal and 60°incidence (80–100%). Also note the very high reflectance of lightpolarized in the stretched direction in the visible range (400–700 nm)shown by curve c. Reflectance is nearly 99% between 500 and 650 nm.

EXAMPLE 5

(PEN:sPS, 481, Polarizer)

A 481 layer multilayer film was made from a polyethylene naphthalate(PEN) with an intrinsic viscosity of 0.56 dl/g measured in 60 wt. %phenol and 40 wt % dichlorobenzene purchased from Eastman Chemicals anda syndiotactic polystyrene (sPS) homopolymer (weight average molecularweight=200,000 Daltons, sampled from Dow Corporation). The PEN was onthe outer layers and was extruded at 26 pounds per hour and the sPS at23 pounds per hour. The feedblock used produced 61 layers with each ofthe 61 being approximately the same thickness. After the feedblock three(2×) multipliers were used. Equal thickness skin layers containing thesame PEN fed to the feedblock were added after the final multiplier at atotal rate of 22 pounds per hour. The web was extruded through a 12″wide die to a thickness of about 0.011 inches (0.276 mm). The extrusiontemperature was 290° C.

This web was stored at ambient conditions for nine days and thenuniaxially oriented on a tenter. The film was preheated to about 320° F.(160° C.) in about 25 seconds and drawn in the transverse direction to adraw ratio of about 6:1 at a rate of about 28% per second. No relaxationwas allowed in the stretched direction. The finished film thickness wasabout 0.0018 inches (0.046 mm).

FIG. 7 shows the optical performance of this PEN:sPS reflectivepolarizer containing 481 layers. Curve a shows transmission of lightpolarized in the non-stretch direction at normal incidence, curve bshows transmission of p-polarized light at 60° incidence, and curve cshows transmission of light polarized in the stretch direction at normalincidence. Note the very high transmission of p-polarized light at bothnormal and 60° incidence. Average transmission for curve a over 400–700nm is 86.2%, the average transmission for curve b over 400–700 nm is79.7%. Also note the very high reflectance of light polarized in thestretched direction in the visible range (400–700 nm) shown by curve c.The film has an average transmission of 1.6% for curve c between 400 and700 nm. The % RMS color for curve a is 3.2%, while the % RMS color forcurve b is 18.2%.

EXAMPLE 6

(PEN:coPEN, 603, Polarizer)

A reflecting polarizer comprising 603 layers was made on a sequentialflat-film making line via a coextrusion process. A polyethylenenaphthalate (PEN) with an intrinsic viscosity of 0.47 dl/g (in 60 wt %phenol plus 40 wt % dichlorobenzene) was delivered by an extruder at arate of 83 pounds (38 kg) per hour and the CoPEN was delivered byanother extruder at 75 pounds (34 kg) per hour. The CoPEN was acopolymer of 70 mole %, 2,6 naphthalene dicarboxylate methyl ester, 15mole % dimethyl terephthalate, and 15 mole % dimethyl isophthalate withethylene glycol. The feedblock method was used to generate 151 layers.The feedblock was designed to produce a stack of films having athickness gradient from top to bottom, with a thickness ratio of 1.22from the thinnest layers to the thickest layers. This optical stack wasmultiplied by two sequential multipliers. The nominal multiplicationratio of the multipliers was 1.2 and 1.4, respectively. Between thefinal multiplier and the die, skin layers were added composed of thesame CoPEN described above, delivered by a third extruder at a totalrate of 106 pounds (48 kg) per hour. The film was subsequently preheatedto 300° F. (150° C.) in about 30 seconds and drawn in the transversedirection to a draw ratio of approximately 6 at an initial rate of about20% per second. The finished film thickness was approximately 0.0035inch (0.089 mm).

FIG. 7B shows the optical performance of the polarizer of Example 6.Curve a shows transmission of light polarized in the non-stretchdirection at normal incidence, curve b shows transmission of p-polarizedlight in the nonstretch direciton at 50 degree angle of incidence, andcurve c shows transmission of light polarized in the stretch directionat normal incidence. Note the very high transmission of light polarizedin the non-stretch direction. Average transmission for curve a over400–700 nm is 87%. Also note the very high reflectance of lightpolarized in the stretched direction in the visible range (400–700 nm)shown by curve c. The film has an average transmission of 2.5% for curvec between 400 and 700 nm. The % RMS color for curve b is 5%.

II. Optical Devices Using Multilayer Optical Films

Optical devices according to the present invention use multilayeroptical films to polarize and/or reflect light. The advantages of usingmultilayer optical film in optical devices involving reflection of lightare graphically illustrated in FIG. 8. Curve a shows the totalreflectivity as a function of the number of reflections for conventionreflector that has 96% reflectivity (i.e., about 4% of the light isabsorbed at each reflection). As shown by curve a, the intensity oflight which has been reflected decreases significantly after arelatively low number of reflections when the surface reflecting thelight absorbs only about 4% of the light. In contrast, curve b shows thetotal reflectivity for a multilayer mirror film having a reflectivity ofabout 99.4%. Curve b clearly shows a much smaller decrease in totalreflectivity. The difference becomes especially pronounced after only2–4 reflections.

For example, for five reflections, the intensity of light is about 97%for light reflected from multilayer optical films according to thepresent invention, while the intensity drops to about 81.5% for lightreflected from a conventional reflector which is only about 3.5% lessefficient. Although it is difficult to determine the average number ofreflections experienced by light in a backlight system, the number ofreflections can be expected to increase as aspect ratio (defined morecompletely below) increase in any given backlight system. Thoseincreased reflections would cause a significant loss in efficiency forbacklight systems using conventional reflectors which would not beexperienced in backlight systems employing multilayer optical filmreflectors according to the present invention.

The practical value of this characteristic is that the efficiency of thepresent optical device is greatly enhanced as compared to systemsemploying conventional reflectors. Stated another way, the number ofacceptable reflections for a given light ray in optical devicesemploying multilayer optical film according to the present invention canbe significantly increased without substantially impairing the overalloutput of the device as compared to optical devices employing knownreflectors/polarizers. This means that the present optical devices canbe used to transmit and transport light over greater distances withbetter efficiency than presently known conventional reflectors.

Optical devices which incorporate the multilayer optical film can bemost generally described as devices in which at least a portion of thelight entering and/or exiting the device is reflected from an opticalsurface comprising the multilayer optical film. For the purpose of thisinvention, an “optical surface” will be defined as a surface, planar orotherwise, which reflects at least a portion of randomly polarized lightincident upon it. More preferably, at least a portion of the lighttraveling through the optical devices will be reflected from an opticalsurface more than once, thereby exploiting the advantages of themultilayer optical film.

A subset of optical devices incorporating multilayer optical filmaccording to the present invention will comprise two or more opticalsurfaces and can generally be categorized into devices in which theoptical surfaces are arranged in a parallel or a non-parallel opposingarrangement.

Optical devices with substantially parallel optical surfaces include,but are not limited to: light pipes, light boxes, rectangular lightguides, etc. For those devices designed to transmit light from onelocation to another, such as a light pipe, it is desirable that theoptical surfaces absorb and transmit a minimal amount of light incidentupon them while reflecting substantially all of the light. In otherdevices such as light boxes and light guides, it may be desirable todeliver light to a selected area using generally reflective opticalsurfaces and to then allow for transmission of light out of the devicein a known, predetermined manner. In such devices, it may be desirableto provide a portion of the optical surface as partially reflective toallow light to exit the device in a predetermined manner. Examples ofsuch devices will be described more completely below.

Another class of optical devices which include two or more reflectiveoptical surfaces are devices in which the reflective optical surfacesconverge towards each other as distance from a light source (or point ofentry into the device) increases. This construction is especially usefulin optical devices where it is desired to return light emitted from anoptical source towards the direction from which the light entered thedevice. Optical devices with converging reflective optical surfaces willtypically reflect a majority of light in a direction generally towardsthe source of the light.

Yet another class of optical devices which include two or more opticalsurfaces are devices in which the reflective optical surfaces diverge asdistance from a light source (or point of entry into the device)increases. Optical devices with diverging reflective optical surfaceswill typically tend to collimate light. The amount and degree ofcollimation will depend on the location of the light source relative tothe narrow end of the device and the rate of divergence of themultilayer reflective optical film surfaces.

In a preferred embodiment, the optical devices are hollow as this willtend to decrease the amount of absorption at each reflection as light istransported by the optical devices.

In the effort to direct light towards a specific target, such as in tasklighting, solar collectors, or otherwise, it may be preferred that thediverging optical surfaces form a parabola or cone. If a parabolic shapeis used, collimation is best accomplished for light passing through oremanating from the focal point of the parabola. The specifics ofdesigning the shape of such devices will be well known to those skilledin the art and will not be discussed herein.

Turning now to the figures in which illustrative examples of opticaldevices according to the present invention are depicted, FIGS. 9 and 10depict one illustrative optical device 110 in a cross-sectionalschematic view in FIG. 9 and a perspective view in FIG. 10. Opticaldevice 110 is commonly referred to as a light box and can besubstantially rectangular as shown or it can take any other shapedesired based on aesthetics or functional considerations. Light boxesare typically substantially enclosed volumes in which one or more lightsources are located. The volume is preferably lined with a reflectivesurface and includes either partially reflective areas or voids whichallow light to escape from the light box in a predetermined pattern ormanner.

The illustrative light box 110 depicted in FIGS. 9 and 10 includes atleast two opposing reflective and/or partially reflective opticalsurfaces 112 and 114 comprised of the multilayer optical film. It ismost preferred that all of the interior reflective surfaces of the lightbox 110 are covered by the multilayer optical film. By using themultilayer optical film according to the present invention for all ofthe reflective surfaces within the light box 110, absorption losses canbe greatly reduced as compared to devices using conventional reflectorsand/or polarizers. In some instances, however, all or a portion ofeither or both optical surfaces 112 and 114 can be constructed fromother materials.

Where multilayer optical film is used in any optical device, it will beunderstood that it can be laminated to a support (which itself may betransparent, opaque reflective or any combination thereof) or it can beotherwise supported using any suitable frame or other support structurebecause in some instances the multilayer optical film itself may not berigid enough to be self-supporting in an optical device such asillustrative device 110.

The optical device 110 illustrated in FIG. 9 includes two light sources118 a and 118 b, referred to commonly as 118, which emit light into theinterior of the device 110. Light emitted from the sources 118 willtypically reflect between surfaces 112 and 114 numerous times beforeexiting the device 110 through a partially reflective area ortransmissive void located in surface 112, denoted by reference number130 in FIG. 10.

For illustration purposes, light rays 120 and 122 are shown as emanatingfrom source 118 a and reflecting within the optical device 110 untilthey exit from areas such as 130 in layer 112. In an illuminated signdepicted as the illustrative optical device 110, areas 130 willtypically comprise advertising or other informational messages or,alternatively, may comprise a decorative display of some type. Althoughonly areas 130 are depicted as transmitting light through opticalsurface 112, it will be understood that all or any portion of bothsurfaces 112 and 114 may transmit light out of device 110.

Areas 130 which transmit light can be provided of many differentmaterials or constructions. The areas 130 can be made of multilayeroptical film or any other transmissive or partially transmissivematerials. One way to allow for light transmission through areas 130 isto provide areas in optical surface 112 which are partially reflectiveand partially transmissive. Partial reflectivity can be imparted tomultilayer optical films in areas 130 according to the present inventionby a variety of means.

In one aspect, areas 130 may comprise multi-layered optical film whichis uniaxially stretched to allow transmission of light having one planeof polarization while reflecting light having a plane of polarizationorthogonal to the transmitted light. Rays 120 a and 120 b of light asdepicted in FIG. 9 illustrate such a situation in which light having onepolarization direction is transmitted through multi-layered optical film130 while light having the orthogonal polarization direction isreflected back into optical device 110.

When areas 130 are provided from a multilayer reflective polarizingfilm, it is preferable that the optical device 110 include somemechanism for randomizing polarization orientation of the lightreflected back into the interior of the device 110. One mechanism forrandomizing polarization orientation would be to provide a thinpigmented coating on optical surface 114 to randomize polarization andscatter light reflected from the areas 130. Another mechanism is to adda birefringent polymer film, or to have a birefringent skin layer on theMOF mirror. Any mechanism, however, by which the polarizationorientation of returned light 120 b can be modified after reflectionfrom the reflective polarizing areas 130 is desirable as it can then bereturned to areas 130 and, theoretically, a portion of the light willthen have the proper polarization orientation to allow transmissionthrough areas 130 and out of optical device 110.

Light ray 122 depicts the effect of an alternate means of providing fortransmission of light through areas 130 in an optical device 110according to the present invention. Light ray 122 is transmitted throughareas 130 without reflection through a void formed in the opticalsurface 112. As a result, there is no partial reflection of light ray122 as opposed to light ray 120 as described above. In this situation,optical surface 112 is itself substantially completely reflective,except for those voids in areas 130 which transmit light withoutsubstantial reflection.

It will be understood that the term “void” can be used to describe anactual physical aperture through optical surface 112 as well as clear ortransparent areas formed in the optical surface 112 which do notsubstantially reflect light. The number and size of multiple aperturesin area 130 of optical surface 112 may be varied to control the amountof light transmitted through the areas 130. At one extreme, areas 130may even constitute complete voids in optical surface 112, althoughlarge voids are typically undesirable to protect the interior of thedevice 110 from debris, dust, etc.

An alternate embodiment of an optical device 110 can be provided whereat least the areas 130 in optical surface 112 do not comprise amultilayer optical film at all, but rather comprise a different class ofpartially reflective films, such as a structured partially reflectivefilm. Exemplary micro-replicated structured partially reflective filmsare marketed as Optical Lighting Film, and Brightness Enhancement Film,available from Minnesota Mining and Manufacturing Company, St. Paul,Minn.

In those instances where a less efficient multilayer optical film isused (i.e., some of the light incident upon the multilayer optical filmsurfaces is lost through transmission), it may be advantageous toprovide the back surfaces of the multilayer optical film, i.e., thesurface facing the exterior of the device 110, with a thin metal orother reflective coating to reflect light that would otherwise be lostto transmission, thereby improving the reflectivity of the multilayeroptical film. It will of course, be understood that the metallic orother reflective coating may suffer from some absorption losses, but thefraction of light transmitted through the film will typically be lessthan 5% (more preferably less than 1%) of the total light incident onthe film. The metallic or other reflective coating may also be useful toreduce visible iridescence if leakage of narrow bands of wavelengthsoccurs in the multilayer optical film. In general, however, the highefficiency multilayer reflective films are preferred.

Due to the high efficiency of the multilayer optical film in reflectinglight in optical devices 110, the number and intensity of light sources118 needed to provide uniform illumination over the areas 130 can bereduced. Any optical device design can be less concerned about thenumber of reflections a light ray will make within device 110 beforeexiting as illustrated in FIG. 10 and described above.

Aspect ratio in a device 110 is typically determined by comparing thedepth of the light box, indicated as D in FIG. 10 to the length andheight of the device 110, indicated as L and H, respectively. In someinstances, aspect ratio may be the ratio of depth D as compared to thearea which is defined by the length times the height of optical device110.

FIG. 11 is a schematic cross-sectional representation of a convergingwedge optical device 210, according to the present invention,incorporating multilayer optical film. In any optical device employing aconverging wedge design, the optical surfaces 212 and 214 are arrangedin a converging relationship in which the optical surfaces convergetowards each other as distance from the opening 211 into the device 210increases. In the preferred embodiment, the surfaces 212 and 214 arecomprised of a multilayer optical film. Also, the optical device 210 ispreferably hollow to minimize absorption losses.

It will be understood that the optical device 210 could comprise twogenerally planar optical surfaces 212 and 214. One specific example of aconverging wedge design would be a light guide used in a backlightassembly for a liquid crystal display device. Another specific exampleof an optical device represented in FIG. 11 could comprise a generallyconical device having a cross-section taken along the longitudinal axisof the device 210. In a conical device, optical surfaces 212 and 214 mayactually be portions of a continuous surface which appears discontinuousdue to the cross-sectional nature of the view in FIG. 11.

A light ray 220 is depicted as entering the optical device 210 throughopening 211 as shown and is reflected numerous times before exiting ingenerally the same direction from which it entered the device 210.Optical surfaces 212 and 214 could be comprised of many differentmaterials. For example both surfaces 212 and 214 could be comprised ofmultilayer optical films according to the present invention and aportion or all of either or both surfaces 212 and 214 could becompletely reflective or partially reflective.

If a less efficient multilayer optical film is used for reflectiveoptical surfaces 212 and 214 and it is desired that both surfacesprevent transmission of light, they can be coated on their “exterior”surfaces with a reflective coating such as a thin metallic layer orother reflective coating. That additional layer will help to ensure thatlayers 212 and 214 do not transmit light. In some instances, however, itmay be desirable to provide one or both of the multi-layered opticalfilms 212 and 214 as partially reflective to allow some leakage oflight, polarized or not, through surfaces 212 and/or 214 in a uniform orother controlled manner. One specific example of a device 210 whereuniform distribution of light is desired is a light guide backlightassembly for a liquid crystal display.

FIG. 12 is a schematic cross-sectional representation of a divergingwedge optical device 310 according to the present invention. In anyoptical device employing a diverging wedge design, the optical surfaces312 and 314 are arranged in a diverging relationship in which thesurfaces diverge as distance from the light source 318 increases. In apreferred embodiment, the surfaces 312 and 314 are comprised of amultilayer optical film. Also, the optical device 310 is preferablyhollow to minimize absorption losses. It will be understood that, likethe converging wedge device 210, the diverging wedge depicted in FIG. 12could comprise two generally planar optical surfaces 312 and 314 or thatdevice 310 could comprise a generally conical, parabolic or other shapein which the depicted cross-section is taken along the longitudinal axisof device 310. In such an optical device, optical surfaces 312 and 314may actually be portions of a continuous surface which appearsdiscontinuous due to the cross-sectional nature of the view in FIG. 11.

An optical device which includes diverging optical surfaces will tend tocollimate light exiting it as light rays 320 and 322 illustrate. Thedevice 310 depicted in FIG. 12 includes a light source 318 located atthe entry into device 310. It will, however, be understood that adiverging optical device may include a plurality of sources 318. Ifdevice 310 were formed in a roughly parabolic shape, that collimationwould be more pronounced if the light source 318 was located proximatethe focal point of the parabola. Alternatively, a diverging opticaldevice 310 could also rely on a light generated from a source or sourceslocated away from the actual opening into the diverging optical device310.

In general, the degree and amount of collimation of light exiting such adevice 310 is dependent on a number of factors including the angle oflight rays entering the device, the location of the light source, andthe shape and/or angular relationship between the optical surfaces 312and 314.

FIG. 13 is a cross-sectional schematic view of another illustrativeoptical device 410 formed using the multilayer optical film according tothe present invention. The cross-section of device 410 as depicted inFIG. 13 is taken along a longitudinal axis which shows two generallyparallel optical surfaces 412 and 414. An additional view is depicted inFIG. 14, which shows a cross-section of device 410 taken transverse tothe longitudinal axis. As depicted, device 410 has a generally circularshape.

Optical devices such as device 410 are typically used to transmit lightbetween two locations and are commonly referred to as “light pipes.”Such devices have a longitudinal axis and a cross-section transverse tothat axis which forms a closed plane figure. Examples of some typicalcross-section figures include circles (such as that shown in FIG. 14),ellipses, polygons, closed irregular curves, triangles, squares,rectangles or other polygonal shapes. Any device 410 having a closedplane figure transverse cross-section appears as two surfaces in alongitudinal cross-section as shown in FIG. 13 even though the device410 may actually be formed from a single continuous optical surface.

Because the multilayer optical film according to the present inventionused absorbs substantially none of the light incident upon it, lightpipes constructed of multilayer optical film according to the presentinvention can extend for a relatively large distances withoutsignificant loss of throughput.

It is particularly advantageous to use the multilayer optical film withdevices such as light pipes in which a large portion of the lighttravelling through the device approaches the surfaces of the device atshallow angles. Known multilayered polymer reflective films are notefficient at reflecting light approaching them at shallow angles and,therefore, would suffer from large transmissive losses. The presentmultilayer optical film, however, is able to reflect such light with themuch the same efficiency as light approaching the film normal to thesurfaces.

Alternately, it will be understood that, a device such as light pipe 410may include sections which are partially transmissive, thus allowinglight to escape from the device. The transmission mechanisms may includemultilayer reflective polarizing sections, voids or any other mechanismas described with respect to the illustrative embodiments above. Suchdesigns do, however, start to resemble light boxes or guides depictedand described in conjunction with FIGS. 9 and 10, above.

FIG. 15 illustrates another optical device according to the presentinvention. The optical device 505 depicted in FIG. 15 could be used, forexample, in a decorative application such as a flower or a bow. Device505 is constructed of a plurality of multilayer optical film layers(such as layers 510 and 520) connected generally in their centers by apost or some other mechanism. Although the layers are depicted asgenerally circular, it will be understood that many different shapescould be provided.

The layers can be wrinkled or otherwise manipulated to give the device505 volume. The wrinkling of multilayer optical film layers alsoprovides device 510 with multiple converging wedges arranged generallyvertically to return light incident on the device 505 to a viewer.

Although not required, leakage or transmission of light through thelayers of multilayer optical film in device 505 is not a great concernas transmitted light can be reflected out of the device 505 by theadjacent diverging wedge formed by the next layer of film. Because ofthe adjacent diverging wedges in device 505, it makes highly efficientuse of leakage between the vertically arranged converging wedges becauselight escaping one wedge could be reflected back out of the adjacentwedge into which the light is transmitted. As a result, device 505 hasan unusually brilliant appearance.

The multilayer optical film may also be provided in the form ofelongated strips. Such strips of film can be advantageously used to formother configurations of optical devices which can be used, for example,as decorative bows, such as any of those described in U.S. Pat. No.3,637,455 (Pearson et al.); U.S. Pat. No. 4,329,382 (Truskolaski etal.); U.S. Pat. No. 4,476,168 (Aoyama); and U.S. Pat. No. 4,515,837(Chong); and U.S. Pat. No. 5,468,523 (Huss); and U.S. Pat. No. 5,614,274(Huss); the entire disclosures of all of which are incorporated hereinby reference.

Optical device 505 illustrates another significant advantage of theoptical devices incorporating multilayer optical film according to thepresent invention, i.e., that the devices need not exhibit symmetry tobe effective. In fact, optical devices according to the presentinvention need not exhibit symmetry in any plane or about any line butcan still function effectively and efficiently due to the low absorbanceand high reflectance both at normal angles and at high angles away fromthe normal of the multilayer optical films.

Symmetry in optical devices is provided in many instances to reduce orminimize the number of reflections experienced by light travellingthrough the devices. Minimizing reflections is particularly importantwhen using conventional reflectors because of their relatively highabsorptivities (see FIG. 8 and the accompanying description above).Because optical devices using multilayer optical film according to thepresent invention experience significantly reduced absorption, it ismuch less important to minimize the number of reflections and,consequently, symmetry is not as important to maintain the efficiency ofthe optical devices.

As a result, although the illustrative optical devices described abovedo generally exhibit symmetry about at least one axis, the presentinvention should not be limited to optical devices having an axis ofsymmetry. Furthermore, the present invention has been described abovewith respect to illustrative examples to which modifications may be madewithout departing from the scope of the invention as defined by theappended claims.

III. Discussion from U.S. Pat. No. 5,882,774 (Jonza et al.)

The present invention as illustrated in FIGS. 1 a and 1 b includes amultilayered polymeric sheet 10 having alternating layers of acrystalline naphthalene dicarboxylic acid polyester such as 2,6polyethylene naphthalate (PEN) 12 and a selected polymer 14 useful as areflective polarizer or mirror. By stretching PEN/selected polymer overa range of uniaxial to biaxial orientation, a film is created with arange of reflectivities for differently oriented plane-polarizedincident light. If stretched biaxially, the sheet can be stretchedasymmetrically along orthogonal axes or symmetrically along orthogonalaxes to obtain desired polarizing and reflecting properties.

For the polarizer, the sheet is preferably oriented by stretching in asingle direction and the index of refraction of the PEN layer exhibits alarge difference between incident light rays with the plane ofpolarization parallel to the oriented and transverse directions. Theindex of refraction associated with an in-plane axis (an axis parallelto the surface of the film) is the effective index of refraction forplane-polarized incident light whose plane of polarization is parallelto that axis. By oriented direction is meant the direction in which thefilm is stretched. By transverse direction is meant that directionorthogonal in the plane of the film to the direction in which the filmis oriented.

PEN is a preferred material because of its high positive stress opticalcoefficient and permanent birefringence after stretching, with therefractive index for polarized incident light of 550 nm wavelengthincreasing when the plane of polarization is parallel to the stretchdirection from about 1.64 to as high as about 1.9. The differences inrefractive indices associated with different in-plane axes exhibited byPEN and a 70-naphthalate/30-terephthalate copolyester (coPEN) for a 5:1stretch ratio are illustrated in FIG. 2 of U.S. Pat. No. 5,882,774(Jonza et al.). In that figure, the data on the lower curve representthe index of refraction of PEN in the transverse direction and the coPENwhile the upper curve represents the index of refraction of PEN in thestretch direction. PEN exhibits a difference in refractive index of 0.25to 0.40 in the visible spectrum. The birefringence (difference inrefractive index) can be increased by increasing the molecularorientation. PEN is heat stable from about 155° C. up to about 230° C.depending upon shrinkage requirements of the application. Although PENhas been specifically discussed above as the preferred polymer for thebirefringent layer, polybutylene naphthalate is also a suitable materialas well as other crystalline naphthalene dicarboxylic polyesters. Thecrystalline naphthalene dicarboxylic polyester should exhibit adifference in refractive indices associated with different in-plane axesof at least 0.05 and preferably above 0.20.

Minor amounts of comonomers may be substituted into the naphthalenedicarboxylic acid polyester so long as the high refractive index in thestretch direction(s) is not substantially compromised. A drop inrefractive index (and therefore decreased reflectivity) may be counterbalanced by advantages in any of the following: adhesion to the selectedpolymer layer, lowered temperature of extrusion, better match of meltviscosities, better match of glass transition temperatures forstretching. Suitable monomers include those based on isophthalic,azelaic, adipic, sebacic, dibenzoic, terephthalic, 2,7- naphthalenedicarboxylic, 2,6-naphthalene dicarboxylic or cyclohexanedicarboxylicacids.

The PEN/selected polymer resins of the present invention preferably havesimilar melt viscosities so as to obtain uniform multilayer coextrusion.The two polymers preferably have a melt viscosity within a factor of 5at typical shear rates.

The PEN and the preferred selected polymer layers of the presentinvention exhibit good adhesion properties to each other while stillremaining as discrete layers within the multilayered sheet.

The glass transition temperatures of the polymers of the presentinvention are compatible so adverse effects such as cracking of one setof polymer layers during stretching does not occur. By compatible ismeant that the glass transition temperature of the selected polymer islower than the glass transition temperature of the PEN layer. The glasstransition temperature of the selected polymer layer temperature may beslightly higher than the glass transition temperature of the PEN layer,but by no more than 40° C.

Preferably, the layers have a ¼ wavelength thickness with different setsof layers designed to reflect different wavelength ranges. Each layerdoes not have to be exactly ¼ wavelength thick. The overridingrequirement is that the adjacent low-high index film pair have a totaloptical thickness of 0.5 wavelength. The bandwidth of a 50-layer stackof PEN/coPEN layers having the index differential indicated in FIG. 2 ofU.S. Pat. No. 5,882,774 (Jonza et al.), with layer thicknesses chosen tobe a ¼ wavelength of 550 nm, is about 50 nm. This 50-layer stackprovides roughly a 99 percent average reflectivity in this wavelengthrange with no measurable absorption. A computer-modeled curve showingless than 1 percent transmission (99 percent reflectivity) isillustrated in FIG. 3 of U.S. Pat. No. 5,882,774 (Jonza et al.). FIGS.3–8 of that patent include data characterized as percent transmission.It should be understood that since there is no measurable absorbance bythe film of the present invention that percent reflectivity isapproximated by the following relationship:100−(percent transmission)=(percent reflectivity).

The preferred selected polymer layer 14 remains isotropic in refractiveindex and substantially matches the refractive index of the PEN layerassociated with the transverse axis as illustrated in FIG. 1 a. Lightwith its plane of polarization in this direction will be predominantlytransmitted by the polarizer while light with its plane of polarizationin the oriented direction will be reflected as illustrated in FIG. 1 b.

The reflective polarizer of the present invention is useful in opticalelements such as ophthalmic lenses, mirrors and windows. The polarizeris characterized by a mirror-like look which is considered stylish insunglasses. In addition, PEN is a very good ultraviolet filter,absorbing ultraviolet efficiently up to the edge of the visiblespectrum. The reflective polarizer of the present invention would alsobe useful as a thin infrared sheet polarizer.

For the polarizer, the PEN/selected polymer layers have at least oneaxis for which the associated indices of refraction are preferablysubstantially equal. The match of refractive indices associated withthat axis, which typically is the transverse axis, results insubstantially no reflection of light in that plane of polarization. Theselected polymer layer may also exhibit a decrease in the refractiveindex associated with the stretch direction. A negative birefringence ofthe selected polymer has the advantage of increasing the differencebetween indices of refraction of adjoining layers associated with theorientation axis while the reflection of light with its plane ofpolarization parallel to the transverse direction is still negligible.Differences between the transverse-axis-associated indices of refractionof adjoining layers after stretching should be less than 0.05 andpreferably less than 0.02. Another possibility is that the selectedpolymer exhibits some positive birefringence due to stretching, but thiscan be relaxed to match the refractive index of the transverse axis ofthe PEN layers in a heat treatment. The temperature of this heattreatment should not be so high as to relax the birefringence in the PENlayers.

The preferred selected polymer for the polarizer of the presentinvention is a copolyester of the reaction product of a naphthalenedicarboxylic acid or its ester such as dimethyl naphthalate ranging from20 mole percent to 80 mole percent and isophthalic or terephthalic acidor their esters such as dimethyl terephthalate ranging from 20 molepercent to 80 mole percent reacted with ethylene glycol. Othercopolyesters within the scope of the present invention have theproperties discussed above and have a refractive index associated withthe transverse axis of approximately 1.59 to 1.69. Of course, thecopolyester must be coextrudable with PEN. Other suitable copolyestersare based on isophthalic, azelaic, adipic, sebacic, dibenzoic,terephthalic, 2,7- naphthalene dicarboxylic, 2,6-naphthalenedicarboxylic or cyclohexanedicarboxylic acids. Other suitable variationsin the copolyester include the use of ethylene glycol, propane diol,butane diol, neopentyl glycol, polyethylene glycol, tetramethyleneglycol, diethylene glycol, cyclohexanedimethanol, 4-hydroxy diphenol,propane diol, bisphenol A, and 1,8-dihydroxy biphenyl, or1,3-bis(2-hydroxyethoxy)benzene as the diol reactant. A volume averageof the refractive indices of the monomers would be a good guide inpreparing useful copolyesters. In addition, copolycarbonates having aglass transition temperature compatible with the glass transitiontemperature of PEN and with a refractive index associated with thetransverse axis of approximately 1.59 to 1.69 are also useful as aselected polymer in the present invention. Formation of the copolyesteror copolycarbonate by transesterification of two or more polymers in theextrusion system is another possible route to a viable selected polymer.

To make a mirror, two uniaxially stretched polarizing sheets 10 arepositioned with their respective orientation axes rotated 90°, or thesheet 10 is biaxially stretched. In the latter case, both PEN refractiveindices in the plane of the sheet increase and the selected polymershould be chosen with as low of a refractive index as possible toreflect light of both planes of polarization. Biaxially stretching themultilayered sheet will result in differences between refractive indicesof adjoining layers for planes parallel to both axes thereby resultingin reflection of light in both planes of polarization directions.Biaxially stretching PEN will increase the refractive indices associatedwith those axes of elongation from 1.64 to only 1.75, compared to theuniaxial value of 1.9. Therefore to create a dielectric mirror with 99percent reflectivity (and thus with no noticeable iridescence) a lowrefractive index coPET is preferred as the selected polymer. Opticalmodeling indicates this is possible with an index of about 1.55. A300-layer film with a 5 percent standard deviation in layer thickness,designed to cover half of the visible spectrum with six overlappingquarterwave stacks, has the predicted performance shown in FIG. 4. Agreater degree of symmetry of stretching yields an article that exhibitsrelatively more symmetric reflective properties and relatively lesspolarizing properties.

If desired, two or more sheets of the invention may be used in acomposite to increase reflectivity, optical band width, or both. If theoptical thicknesses of pairs of layers within the sheets aresubstantially equal, the composite will reflect, at somewhat greaterefficiency, substantially the same band width and spectral range ofreflectivity (i.e., “band”) as the individual sheets. If the opticalthicknesses of pairs of layers within the sheets are not substantiallyequal, the composite will reflect across a broader band width than theindividual sheets. A composite combining mirror sheets with polarizersheets is useful for increasing total reflectance while still polarizingtransmitted light. Alternatively, a single sheet may be asymmetricallybiaxially stretched to produce a film having selective reflective andpolarizing properties.

The preferred selected polymer for use in a biaxially stretched mirrorapplication is based on terephthalic, isophthalic, sebacic, azelaic orcyclohexanedicarboxylic acid to attain the lowest possible refractiveindex while still maintaining adhesion to the PEN layers. Naphthalenedicarboxylic acid may still be employed in minor amounts to improve theadhesion to PEN. The diol component may be taken from any that have beenpreviously mentioned. Preferably the selected polymer has an index ofrefraction of less than 1.65 and more preferably an index of refractionof less than 1.55.

It is not required that the selected polymer be a copolyester orcopolycarbonate. Vinyl polymers and copolymers made from monomers suchas vinyl naphthalenes, styrenes, ethylene, maleic anhydride, acrylates,methacrylates, might be employed. Condensation polymers other thanpolyesters and polycarbonates might also be useful, examples include:polysulfones, polyamides, polyurethanes, polyamic acids, polyimides.Naphthalene groups and halogens such as chlorine, bromine and iodine areuseful in increasing the refractive index of the selected polymer to thedesired level (1.59 to 1.69) to substantially match the refractive indexof PEN associated with the transverse direction for a polarizer.Acrylate groups and fluorine are particularly useful in decreasingrefractive index for use in a mirror.

The relationships between the indices of refraction in each film layerto each other and to those of the other layers in the film stackdetermine the reflectance behavior of the multilayer stack at any angleof incidence, from any azimuthal direction. Assuming that all layers ofthe same material have the same indices, then a single interface of atwo component quarterwave stack can be analyzed to understand thebehavior of the entire stack as a function of angle.

For simplicity of discussion, therefore, the optical behavior of asingle interface will be described. It shall be understood, however,that an actual multilayer stack according to the principles describedherein could be made of tens, hundreds or thousands of layers. Todescribe the optical behavior of a single interface, such as the oneshown in FIG. 2, the reflectivity as a function of angle of incidencefor s and p polarized light for a plane of incidence including thez-axis and one in-plane optic axis will be plotted.

FIG. 2 shows two material film layers forming a single interface, withboth immersed in an isotropic medium of index no. For simplicity ofillustration, the present discussion will be directed toward anorthogonal multilayer birefringent system with the optical axes of thetwo materials aligned, and with one optic axis (z) perpendicular to thefilm plane, and the other optic axes along the x and y axis. It shall beunderstood, however, that the optic axes need not be orthogonal, andthat nonorthogonal systems are well within the spirit and scope of thepresent invention. It shall be further understood that the optic axesalso need not be aligned with the film axes to fall within the intendedscope of the present invention.

The reflectivity of a dielectric interface varies as a function of angleof incidence, and for isotropic materials, is different for p and spolarized light. The reflectivity minimum for p polarized light is dueto the so called Brewster effect, and the angle at which the reflectancegoes to zero is referred to as Brewster's angle.

The reflectance behavior of any film stack, at any angle of incidence,is determined by the dielectric tensors of all films involved. A generaltheoretical treatment of this topic is given in the text by R. M. A.Azzam and N. M. Bashara, “Ellipsometry and Polarized Light”, publishedby North-Holland, 1987.

The reflectivity for a single interface of a system is calculated bysquaring the absolute value of the reflection coefficients for p and spolarized light, given by equations 1 and 2, respectively. Equations 1and 2 are valid for uniaxial orthogonal systems, with the axes of thetwo components aligned.

${\left. {{{\left. 1 \right)\mspace{14mu} r_{p}} = \frac{{n_{2z}*n_{20}\sqrt{n_{1z}^{2} - {n_{o}^{2}{Sin}^{2}\theta}}} - {n_{1z}*n_{10}\sqrt{n_{2z}^{2} - {n_{o}^{2}{Sin}^{2}\theta}}}}{{n_{2z}*n_{20}\sqrt{n_{1z}^{2} - {n_{o}^{2}{Sin}^{2}\theta}}} + {n_{1z}*n_{10}\sqrt{n_{2z}^{2} - {n_{o}^{2}{Sin}^{2}\theta}}}}}2} \right)\mspace{14mu} r_{s}} = \frac{\sqrt{n_{10}^{2} - {n_{o}^{2}{Sin}^{2}\theta}} - \sqrt{n_{20}^{2} - {n_{o}^{2}{Sin}^{2}\theta}}}{\sqrt{n_{10}^{2} - {n_{o}^{2}{Sin}^{2}\theta}} + \sqrt{n_{20}^{2} - {n_{o}^{2}{Sin}^{2}\theta}}}$where θ is measured in the isotropic medium.

In a uniaxial birefringent system, n1x=n1y=n1o, and n2x=n2y=n2o.

For a biaxial birefringent system, equations 1 and 2 are valid only forlight with its plane of polarization parallel to the x-z or y-z planes,as defined in FIG. 10. So, for a biaxial system, for light incident inthe x-z plane, n1o=n1x and n2o=n2x in equation 1 (for p-polarizedlight), and n1o=n1y and n2o=n2y in equation 2 (for s-polarized light).For light incident in the y-z plane, n1o=n1y and n2o=n2y in equation 1(for p-polarized light), and n1o=n1x and n2o=n2x in equation 2 (fors-polarized light).

Equations 1 and 2 show that reflectivity depends upon the indices ofrefraction in the x, y (in-plane) and z directions of each material inthe stack. In an isotropic material, all three indices are equal, thusnx=ny=nz. The relationship between nx, ny and nz determine the opticalcharacteristics of the material. Different relationships between thethree indices lead to three general categories of materials: isotropic,uniaxially birefringent, and biaxially birefringent. Equations 1 and 2describe biaxially birefringent cases only along the x- or y-axis, andthen only if considered separately for the x and y directions.

A uniaxially birefringent material is defined as one in which the indexof refraction in one direction is different from the indices in theother two directions. For purposes of the present discussion, theconvention for describing uniaxially birefringent systems is for thecondition nx=ny≠nz. The x -and y-axes are defined as the in-plane axesand the respective indices, nx and ny, will be referred to as thein-plane indices.

One method of creating a uniaxial birefringent system is to biaxiallystretch (e.g., stretch along two dimensions) a multilayer stack in whichat least one of the materials in the stack has its index of refractionaffected by the stretching process (e.g., the index either increases ordecreases). Biaxial stretching of the multilayer stack may result indifferences between refractive indices of adjoining layers for planesparallel to both axes thus resulting in reflection of light in bothplanes of polarization.

A uniaxial birefringent material can have either positive or negativeuniaxial birefringence. Positive uniaxial birefringence occurs when thez-index is greater than the in-plane indices (nz>nx and ny). Negativeuniaxial birefringence occurs when the z-index is less than the in-planeindices (nz<nx and ny).

A biaxial birefringent material is defined as one in which the indicesof refraction in all three axes are different, e.g., nx≠ny≠nz. Again,the nx and ny indices will be referred to as the in-plane indices. Abiaxial birefringent system can be made by stretching the multilayerstack in one direction. In other words the stack is uniaxiallystretched. For purposes of the present discussion, the x direction willbe referred to as the stretch direction for biaxial birefringent stacks.

Uniaxial Birefringent Systems (Mirrors)

The optical properties and design considerations of uniaxialbirefringent systems will now be discussed. As discussed above, thegeneral conditions for a uniaxial birefringent material are nx=ny≠nz.Thus if each layer 102 and 104 in FIG. 2 is uniaxially birefringent,n1x=n1y and n2x=n2y. For purposes of the present discussion, assume thatlayer 102 has larger in-plane indices than layer 104, and that thusn1>n2 in both the x and y directions. The optical behavior of a uniaxialbirefringent multilayer system can be adjusted by varying the values ofn1z and n2z to introduce different levels of positive or negativebirefringence. The relationship between the various indices ofrefraction can be measured directly, or, the general relationship may beindirectly observed by analysis of the spectra of the resulting film asdescribed herein.

In the case of mirrors, the desired average transmission for light ofeach polarization and plane of incidence generally depends upon theintended use of the mirror. The average transmission along each stretchdirection at normal incidence for a narrow bandwidth mirror across a 100nm bandwidth within the visible spectrum is desirably less than 30%,preferably less than 20% and more preferably less than 10%. A desirableaverage transmission along each stretch direction at normal incidencefor a partial mirror ranges anywhere from, for example, 10% to 50%, andcan cover a bandwidth of anywhere between, for example, 100 nm and 450nm, depending upon the particular application. For a high efficiencymirror, average transmission along each stretch direction at normalincidence over the visible spectrum (400–700 nm) is desirably less than10%, preferably less than 5%, more preferably less than 2%, and evenmore preferably less than 1%. In addition, asymmetric mirrors may bedesirable for certain applications. In that case, average transmissionalong one stretch direction may be desirably less than, for example,50%, while the average transmission along the other stretch directionmay be desirably less than, for example 20%, over a bandwidth of, forexample, the visible spectrum (400–700 nm), or over the visible spectrumand into the near infrared (e.g, 400–850 nm).

Equation 1 described above can be used to determine the reflectivity ofa single interface in a uniaxial birefringent system composed of twolayers such as that shown in FIG. 2. Equation 2, for s polarized light,is identical to that of the case of isotropic films (nx=ny=nz), so onlyequation 1 need be examined. For purposes of illustration, somespecific, although generic, values for the film indices will beassigned. Let n1x=n1y=1.75, n1z=variable, n2x=n2y=1.50, andn2z=variable. In order to illustrate various possible Brewster angles inthis system, no=1.60 for the surrounding isotropic media.

FIG. 16 shows reflectivity versus angle curves for p-polarized lightincident from the isotropic medium to the birefringent layers, for caseswhere n1z is numerically greater than or equal to n2z (n1z≧n2z). Thecurves shown in FIG. 16 are for the following z-index values: a)n1z=1.75, n2z=1.50; b) n1z=1.75, n2z=1.57; c) n1z=1.70, n2z=1.60; d)n1z=1.65, n2z=1.60; e) n1z=1.61, n2z=1.60; and f) n1z=1.60=n2z. As n1zapproaches n2z, the Brewster angle, the angle at which reflectivity goesto zero, increases. Curves a–e are strongly angular dependent. However,when n1z=n2z (curve f), there is no angular dependence to reflectivity.In other words, the reflectivity for curve f is constant for all anglesof incidence. At that point, equation 1 reduces to the angularindependent form: (n2o−n1o)/(n2o+n1o). When n1z=n2z, there is noBrewster effect and there is constant reflectivity for all angles ofincidence.

FIG. 17 shows reflectivity versus angle of incidence curves for caseswhere n1z is numerically less than or equal to n2z. Light is incidentfrom isotropic medium to the birefringent layers. For these cases, thereflectivity monotonically increases with angle of incidence. This isthe behavior that would be observed for s-polarized light. Curve a inFIG. 17 shows the single case for s polarized light. Curves b–e showcases for p polarized light for various values of nz, in the followingorder: b) n1z=1.50, n2z=1.60; c) n1z=1.55, n2z=1.60; d) n1z=1.59,n2z=1.60; and e) n1z=1.60 =n2z. Again, when n1z=n2z (curve e) there isno Brewster effect, and there is constant reflectivity for all angles ofincidence.

FIG. 18 shows the same cases as FIGS. 16 and 17 but for an incidentmedium of index no=1.0 (air). The curves in FIG. 18 are plotted for ppolarized light at a single interface of a positive uniaxial material ofindices n2x=n2y=1.50, n2z=1.60, and a negative uniaxially birefringentmaterial with n1x=n1y=1.75, and values of n1z, in the following order,from top to bottom, of: a) 1.50; b) 1.55; c) 1.59; d) 1.60; f) 1.61; g)1.65; h) 1.70; and i) 1.75. Again, as was shown in FIGS. 16 and 17, whenthe values of n1z and n2z match (curve d), there is no angulardependence to reflectivity.

FIGS. 16, 17, and 18 show that the cross-over from one type of behaviorto another occurs when the z-axis index of one film equals the z-axisindex of the other film. This is true for several combinations ofnegative and positive uniaxially birefringent, and isotropic materials.Other situations occur in which the Brewster angle is shifted to largeror smaller angles.

Various possible relationships between in-plane indices and z-axisindices are illustrated in FIGS. 19, 20, and 21. The vertical axesindicate relative values of indices and the horizontal axes are used toseparate the various conditions. Each Figure begins at the left with twoisotropic films, where the z-index equals the in-plane indices. As oneproceeds to the right, the in-plane indices are held constant and thevarious z-axis indices increase or decrease, indicating the relativeamount of positive or negative birefringence.

The case described above with respect to FIGS. 16, 17, and 18 isillustrated in FIG. 19. The in-plane indices of material one are greaterthan the in-plane indices of material two, material 1 has negativebirefringence (n1z less than in-plane indices), and material two haspositive birefringence (n2z greater than in-plane indices). The point atwhich the Brewster angle disappears and reflectivity is constant for allangles of incidence is where the two z-axis indices are equal. Thispoint corresponds to curve f in FIG. 16, curve e in FIG. 17 or curve din FIG. 18.

In FIG. 20, material one has higher in-plane indices than material two,but material one has positive birefringence and material two hasnegative birefringence. In this case, the Brewster minimum can onlyshift to lower values of angle.

Both FIGS. 19 and 20 are valid for the limiting cases where one of thetwo films is isotropic. The two cases are where material one isisotropic and material two has positive birefringence, or material twois isotropic and material one has negative birefringence. The point atwhich there is no Brewster effect is where the z-axis index of thebirefringent material equals the index of the isotropic film.

Another case is where both films are of the same type, i.e., bothnegative or both positive birefringent. FIG. 21 shows the case whereboth films have negative birefringence. However, it shall be understoodthat the case of two positive birefringent layers is analogous to thecase of two negative birefringent layers shown in FIG. 21. As before,the Brewster minimum is eliminated only if one z-axis index equals orcrosses that of the other film.

Yet another case occurs where the in-plane indices of the two materialsare equal, but the z-axis indices differ. In this case, which is asubset of all three cases shown in FIGS. 19–21, no reflection occurs fors polarized light at any angle, and the reflectivity for p polarizedlight increases monotonically with increasing angle of incidence. Thistype of article has increasing reflectivity for p-polarized light asangle of incidence increases, and is transparent to s-polarized light.This article can be referred to as a “p-polarizer”.

The above described principles and design considerations describing thebehavior of uniaxially birefringent systems can be applied to createmultilayer stacks having the desired optical effects for a wide varietyof circumstances and applications. The indices of refraction of thelayers in the multilayer stack can be manipulated and tailored toproduce devices having the desired optical properties. Many negative andpositive uniaxial birefringent systems can be created with a variety ofin-plane and z-axis indices, and many useful devices can be designed andfabricated using the principles described here.

Biaxial Birefringent Systems (Polarizers)

Referring again to FIG. 2, two component orthogonal biaxial birefringentsystems and the design considerations affecting the resultant opticalproperties will now be described. Again, the system can have manylayers, but an understanding of the optical behavior of the stack isachieved by examining the optical behavior at one interface.

A biaxial birefringent system can be designed to give high reflectivityfor light with its plane of polarization parallel to one axis, for abroad range of angles of incidence, and simultaneously have lowreflectivity and high transmission for light with its plane ofpolarization parallel to the other axis for a broad range of angles ofincidence. As a result, the biaxial birefringent system acts as apolarizer, transmitting light of one polarization and reflecting lightof the other polarization. By controlling the three indices ofrefraction of each film, nx, ny and nz, the desired polarizer behaviorcan be obtained. Again, the indices of refraction can be measureddirectly or can be indirectly observed by analysis of the spectra of theresulting film, as described herein.

Referring again to FIG. 2, the following values to the film indices areassigned for purposes of illustration: n1x=1.88, n1y=1.64, n1z=variable,n2x=1.65, n2y=variable, and n2z=variable. The x direction is referred toas the extinction direction and the y direction as the transmissiondirection.

Equation 1 can be used to predict the angular behavior of the biaxialbirefringent system for two important cases of light with a plane ofincidence in either the stretch (xz plane) or the non-stretch (yz plane)directions. The polarizer is a mirror in one polarization direction anda window in the other direction. In the stretch direction, the largeindex differential of 1.88−1.65=0.23 in a multilayer stack with hundredsof layers will yield very high reflectivities for s-polarized light. Forp-polarized light the reflectance at various angles depends on then1z/n2z index differential.

Reflectivity at off-normal angles, for light with its plane ofpolarization parallel to the transmission axis may be caused by a largez-index mismatch, even if the in-plane y indices are matched. Theresulting system thus has large reflectivity for p, and is highlytransparent to s polarized light. This case was referred to above in theanalysis of the mirror cases as a “p polarizer”.

For uniaxially stretched polarizers, performance depends upon therelationships between the alternating layer indices for all three (x, y,and z) directions. As described herein, it is desirable to minimize they and z index differentials for a high efficiency polarizer.Introduction of a y-index mismatch is describe to compensate for az-index mismatch. Whether intentionally added or naturally occurring,any index mismatch will introduce some reflectivity. An important factorthus is making the x-index differential larger than the y- and z-indexdifferentials. Since reflectivity increases rapidly as a function ofindex differential in both the stretch and non-stretch directions, theratios Δny/Δnx and Δnz/Δnx should be minimized to obtain a polarizerhaving high extinction along one axis across the bandwidth of interestand also over a broad range of angles, while preserving hightransmission along the orthogonal axis. Ratios of less than 0.05, 0.1 or0.25 are acceptable. Ideally, the ratio Δnz/Δnx is 0, but ratios of lessthan 0.25 or 0.5 also produce a useable polarizer.

FIG. 22 shows the reflectivity (plotted as −Log[1−R]) at 75° for ppolarized light with its plane of incidence in the non-stretchdirection, for an 800 layer stack of PEN/coPEN. The reflectivity isplotted as function of wavelength across the visible spectrum (400–700nm). The relevant indices for curve a at 550 nm are n1y=1.64, n1z=1.52,n2y=1.64 and n2z=1.63. The model stack design is a linear thicknessgrade for quarterwave pairs, where each pair thickness is given bydn=do+do(0.003)n. All layers were assigned a random thickness error witha gaussian distribution and a 5% standard deviation.

Curve a shows high off-axis reflectivity across the visible spectrumalong the transmission axis (the y-axis) and that different wavelengthsexperience different levels of reflectivity. This is due to the largez-index mismatch (Δnz=0.11). Since the spectrum is sensitive to layerthickness errors and spatial nonuniformities, such as film caliper, thisgives a biaxial birefringent system with a very nonuniform and“colorful” appearance. Although a high degree of color may be desirablefor certain applications, it is desirable to control the degree ofoff-axis color, and minimize it for those applications requiring auniform, low color appearance, such as liquid crystal displays or othertypes of displays.

Off-axis reflectivity, and off-axis color can be minimized byintroducing an index mismatch to the non-stretch in-plane indices (n1yand n2y) that create a Brewster condition off axis, while keeping thes-polarization reflectivity to a minimum.

FIG. 23 explores the effect of introducing a y-index mismatch inreducing off-axis reflectivity along the transmission axis of a biaxialbirefringent system. With n1z=1.52 and n2z=1.63 (Δnz=0.11), thefollowing conditions are plotted for p polarized light: a) n1y=n2y=1.64;b) n1y=1.64, n2y=1.62; c) n1y=1.64, n2y=1.66. Curve a shows thereflectivity where the in-plane indices n1y and n2y are equal. Curve ahas a reflectance minimum at 0°, but rises steeply after 20°. For curveb, n1y>n2y, and reflectivity increases rapidly. Curve c, where n1y<n2y,has a reflectance minimum at 38°, but rises steeply thereafter.Considerable reflection occurs as well for s polarized light forn1y≠n2y, as shown by curve d. Curves a–d of FIG. 23 indicate that thesign of the y-index mismatch (n1y−n2y) should be the same as the z-indexmismatch (n1z−n2z) for a Brewster minimum to exist. For the case ofn1y=n2y, reflectivity for s polarized light is zero at all angles.

By reducing the z-axis index difference between layers, the off axisreflectivity can be further reduced. If n1z is equal to n2z, FIG. 18indicates that the extinction axis will still have a high reflectivityoff-angle as it does at normal incidence, and no reflection would occuralong the nonstretch axis at any angle because both indices are matched(e.g., n1y=n2y and n1z=n2z).

Exact matching of the two y indices and the two z indices may not bepossible in some multilayer systems. If the z-axis indices are notmatched in a polarizer construction, introduction of a slight mismatchmay be desired for in-plane indices n1y and n2y. This can be done byblending additional components into one or both of the material layersin order to increase or decrease the respective y index. Blending asecond resin into either the polymer that forms the highly birefringentlayers or into the polymer that forms the selected polymer layers may bedone to modify reflection for the transmission axis at normal andoff-normal angles, or to modify the extinction of the polarizer forlight polarized in the extinction axis. The second, blended resin mayaccomplish this by modifying the crystallinity and the index ofrefraction of the polymer layers after orientation.

Another example is plotted in FIG. 24, assuming n1z=1.56 and n2z=1.60(Δnz=0.04), with the following y indices a) n1y=1.64, n2y=1.65; b)n1y=1.64, n2y=1.63. Curve c is for s-polarized light for either case.Curve a, where the sign of the y-index mismatch is the same as thez-index mismatch, results in the lowest off-angle reflectivity.

The computed off-axis reflectance of an 800 layer stack of films at 75°angle of incidence with the conditions of curve a in FIG. 24 is plottedas curve b in FIG. 22. Comparison of curve b with curve a in FIG. 22shows that there is far less off-axis reflectivity, and therefore lowerperceived color and better uniformity, for the conditions plotted incurve b. The relevant indices for curve b at 550 nm are n1y=1.64,n1z=1.56, n2y=1.65 and n2z=1.60.

FIG. 25 shows a contour plot of equation 1 which summarizes the off axisreflectivity discussed in relation to FIG. 2 for p-polarized light. Thefour independent indices involved in the non-stretch direction have beenreduced to two index mismatches, Δnz and Δny. The plot is an average of6 plots at various angles of incidence from 0° to 75° in 15 degreeincrements. The reflectivity ranges from 0.4×10⁻⁴ for contour j, to4.0×10⁻⁴ for contour a, in constant increments of 0.4×10⁻⁴. The plotsindicate how high reflectivity caused by an index mismatch along oneoptic axis can be offset by a mismatch along the other axis.

Thus, by reducing the z-index mismatch between layers of a biaxialbirefringent systems, and/or by introducing a y-index mismatch toproduce a Brewster effect, off-axis reflectivity, and therefore off-axiscolor, are minimized along the transmission axis of a multilayerreflecting polarizer.

Materials Selection and Processing

With the above-described design considerations established, one ofordinary skill will readily appreciate that a wide variety of materialscan be used to form multilayer mirrors or polarizers according to theinvention when processed under conditions selected to yield the desiredrefractive index relationships. The desired refractive indexrelationships can be achieved in a variety of ways, including stretchingduring or after film formation (e.g., in the case of organic polymers),extruding (e.g., in the case of liquid crystalline materials), orcoating. In addition, it is preferred that the two materials havesimilar rheological properties (e.g., melt viscosities) such that theycan be co-extruded.

In general, appropriate combinations may be achieved by selecting, asthe first material, a crystalline or semi-crystalline material,preferably a polymer. The second material, in turn, may be crystalline,semi-crystalline, or amorphous. The second material may have abirefringence opposite to or the same as that of the first material. Or,the second material may have no birefringence.

1. A backlight assembly, comprising: a light guide comprising opposingoptical surfaces, wherein at least a portion of the opposing opticalsurfaces comprises a multilayer optical film that comprises alternatinglayers of at least two materials, wherein the multilayer optical filmreflects light over a wavelength band of interest as a function ofthicknesses of the alternating layers, wherein at least one of thematerials has a stress induced birefringence, and further wherein atleast one opposing optical surface comprises a plurality of voids formedin the multilayer optical film; and at least one light source positionedbetween the opposing optical surfaces, wherein at least one void of theplurality of voids is capable of transmitting light emitted by the atleast one light source.
 2. The assembly of claim 1, wherein themultilayer optical film comprises a mirror.
 3. The assembly of claim 1,wherein at least one void of the plurality of voids comprises a physicalaperture.
 4. The assembly of claim 1, wherein at least one void of theplurality of voids comprises a transparent area.
 5. The assembly ofclaim 1, wherein the multilayer optical film comprises alternating firstand second polymeric materials, wherein the first polymeric material isbirefringent and the second polymeric material is isotropic.
 6. Theassembly of claim 1, wherein the light guide is hollow.
 7. The assemblyof claim 1, wherein the multilayer optical film comprises alternatingfirst and second polymeric materials, wherein the absolute value of thedifference in index of refraction between the first and second polymericmaterials is Δnx along an in-plane direction of the film and is Δnzalong a thickness direction of the film, and wherein Δnx is at least0.05 and Δnz is less than Δnx.
 8. The assembly of claim 1, wherein themultilayer optical film is laminated to a support.
 9. The assembly ofclaim 1, wherein the plurality of voids are uniformly distributed in theopposing optical surface.
 10. The assembly of claim 1, wherein theplurality of voids are randomly distributed in the opposing opticalsurface.
 11. The assembly of claim 1, wherein each void of the pluralityof voids is of equal size.
 12. A backlight assembly, comprising: a lightguide comprising opposing optical surfaces and an opening substantiallyorthogonal to the opposing optical surfaces, wherein at least a portionof the opposing optical surfaces comprises a multilayer optical filmthat comprises alternating layers of at least two materials, wherein themultilayer optical film reflects light over a wavelength band ofinterest as a function of thicknesses of the alternating layers, whereinat least one of the materials has a stress induced birefringence, andfurther wherein at least one opposing optical surface comprises aplurality of voids formed in the multilayer optical film; and at leastone light source positioned proximate the opening, wherein at least onevoid of the plurality of voids is capable of transmitting light emittedby the at least one light source.
 13. The assembly of claim 12, whereinthe multilayer optical film comprises a mirror.
 14. The assembly ofclaim 12, wherein at least one void of the plurality of voids comprisesa physical aperture.
 15. The assembly of claim 12, wherein at least onevoid of the plurality of voids comprises a transparent area.
 16. Theassembly of claim 12, wherein the opposing optical surfaces convergetowards each other as the distance from the opening into the light guideincreases.
 17. The assembly of claim 12, wherein the multilayer opticalfilm comprises alternating first and second polymeric materials, whereinthe first polymeric material is birefringent and the second polymericmaterial is isotropic.
 18. The assembly of claim 12, wherein the lightguide is hollow.
 19. The assembly of claim 12, wherein the multilayeroptical film comprises alternating first and second polymeric materials,wherein the absolute value of the difference in index of refractionbetween the first and second polymeric materials is Δnx along anin-plane direction of the film and is Δnz along a thickness direction ofthe film, and wherein Δnx is at least 0.05 and Δnz is less than Δnx.